WASHINGTON (AP) _ The new president of the College Board urged business executives Thursday to make special efforts to hire minorities if they want to promote better schools.

Donald M. Stewart said that by the end of the century, ''a third of our schoolchildren will be members of minority groups, largely black and Hispanic.''

''Unless our schools are able to educate those youngsters successfully ... it will not merely be a matter of social injustice, but an economic disaster,'' Stewart told the Conference Board, a business research group.

''Leave the doors of economic opportunity open for all people, regardless of race, class or creed. Adopt schools and help to make them better with American business know-how, but hire their graduates as well,'' said Stewart, former president of Spelman College in Atlanta.

''Business has every right to demand top-flight performance by its employees, but it also needs to help in the critically important process of promoting quality schooling and preparation,'' Stewart said.

For minorities, he added, ''this means special efforts, often going under the much maligned title of affirmative action until we get through this terribly difficult period of transition and catch-up.''

''It is our job as educators to make the schools better, but we need your help,'' he said.

Stewart said he was worried that ''it is business as usual in the overwhelming majority of U.S. classrooms'' despite wide-ranging efforts to raise standards in recent years.

Stewart spoke at a Conference Board meeting on ''The New Education Agenda for Business.'' He succeeded George Hanford six weeks ago as president of the New York-based College Board, a non-profit association that sponsors the Scholastic Aptitude Test.